Polyesters are used for a variety of applications, for example, clothing applications, material applications, and medical applications, because of the usefulness of their functions. Particularly, polyethylene terephthalate is superior in general usability and practical utility and is preferably used.
While polyethylene terephthalate is generally produced from terephthalic acid or its ester-forming derivative and ethylene glycol, antimony compounds are used widely as polycondensation catalysts in commercial processes for producing high molecular weight polymers. However, polymers containing antimony compounds have some unfavorable properties as described below.
For example, it is known that when a polymer produced using an antimony catalyst is melt-spun into fibers, a residue of the antimony catalyst deposits around spinneret holes. The reason why the deposit of the residue of the antimony catalyst is formed is considered that an antimony compound in the polymer is denatured in the vicinity of the spinneret and a part of the antimony compound is vaporized and released and then components principally containing antimony remain at the spinneret. Because the progression of such deposition causes defects in filaments, the need for removing the deposit timely arises. Moreover, an antimony catalyst residue in a polymer has an unfavorable drawback that it is prone to form a comparatively large particle and becomes a foreign matter, causing increase in filtering pressure of a filter during the process, filament breakage in spinning, and film rupture in the production of a film and contributes to the reduction in operational efficiency.
Because of such background as described above, polyesters containing no antimony have been demanded. Then, there is known a germanium compound in seeking the role of a polycondensation catalyst in compounds other than antimony compounds, but it is difficult to use germanium compounds widely because they have scarcity value because of their small reserve.
On the other hand, the use of a titanium compound as a catalyst for polymerization is investigated actively for coping with that problem. Since titanium compounds are higher in catalytic activity than antimony compounds, a desired catalytic activity can be achieved by the addition in a small amount, and therefore generation of foreign matter particles or contamination of a spinneret can be inhibited. However, if a titanium compound is used as a polymerization catalyst, side reactions, such as a thermal decomposition reaction and an oxidative decomposition reaction, are also promoted because of the high reactivity of the titanium compound, resulting in a problem that thermal stability is deteriorated and the polymer becomes yellow. That a polymer becomes yellowish is undesirable because this results in deterioration of commercial value when, for example, a polyester is used in the form of fiber, especially fibers for clothing. As described above, in order to obtain a polyester that reduces the generation of foreign matters caused by a catalyst and mold pollution during molding and that has thermal stability and color tone remarkably improved, in comparison to conventional products, it is necessary to solve conflicting problems, i.e., to use a titanium compound as a polymerization catalyst without using an antimony catalyst and to control side reactions without impairing polymerization reaction activity.
On the other hand, there have been proposed as a polymerization catalyst a composition comprising a titanium compound, a phosphorus compound and an amine (patent document 1) and a reaction product resulting from a titanium compound, a phosphorus compound and an aromatic polycarboxylic acid or its anhydride (patent documents 2 to 4). It is possible to reduce foreign matters caused by catalysts by these methods, but resulting polymers do not have satisfactory color tone. Therefore, further improvement of titanium compounds is demanded.
The coloring or the deterioration of heat resistance of a polyester is caused by a side reaction of polyester polymerization as explicitly disclosed in Saturated Polyester Resin Handbook (published by The Nikkan Kogyo Shimbun, Ltd., the first edition, pp. 178-198). In this side reaction of a polyester, a carbonyl oxygen is activated by a metal catalyst having Lewis acidity and thereby beta hydrogen is removed, so that a vinyl terminal group component and an aldehyde component are generated. Such a side reaction triggers coloring of a polymer into yellow color and breakage of ester linkages of the main chain, affording a polymer that is inferior in heat resistance. In particular, when a titanium compound is used as a polymerization catalyst, a large amount of a vinyl terminal group component and an aldehyde component are generated because a side reaction is strongly activated by heat, resulting in a polymer that has been colored in yellow and is inferior in heat resistance. The mechanism of this coloring has not been known perfectly, but it is presumed that a titanium compound and some impurities are coordinated specifically together to produce color. Then, there has been obtained a hypothesis that if a ligand capable of including titanium is used, it is possible to reduce the Lewis acidity of a titanium compound, and therefore the activation of a carbonyl oxygen is inhibited and, as a result, the breakage of ester linkages of the main chain and the generation of vinyl terminal group components and aldehyde components are inhibited, so that the thermal stability of a polymer is good and, moreover, the specific coordination between the titanium compound and impurities can be inhibited and therefore it is possible to inhibit coloring. Then, in aspects of the present invention, as a result of intensive investigations on the basis of this hypothesis for improving the above-mentioned problems, there have been obtained findings that by using mannitol, which is a polyhydric alcohol, as a ligand and making it react with titanium, it is possible to attain the benefits according to exemplary aspects of the present invention. The present invention, according to exemplary embodiments, is specific in the case of using mannitol among polyhydric alcohols, and it was found that a catalyst obtained by making mannitol and a titanium compound react together had a remarkable color tone improving effect. Concretely, it was found that the use a dihydric alcohol, such as ethylene glycol, or a trihydric alcohol, such as glycerol, instead of mannitol failed to exhibit an effect of improving the color tone of a resulting polymer or improvement in thermal stability and mannitol remarkably had these effects. Moreover, it was found that this catalyst was able to reduce foreign matters caused by a catalyst and it not only was superior in the storage stability of a catalyst but also hardly was deactivated even in a polymerization system and was able to greatly reduce a polymerization time. It is presumed that these phenomena are caused by the fact that since a ligand has coordinated to titanium to include it, the compatibility of a titanium compound to PET is increased and the generation of titanium oxide is inhibited which is formed by the reaction of the titanium compound with slightly existing water in the polymerization system.
As a titanium catalyst containing a polyhydric alcohol as a polymerization catalyst, there is a reaction product resulting from a titanium compound, an alcohol having at least two hydroxyl groups, a phosphorus compound, and a base (patent document 5), but the alcohol having two hydroxyl groups as referred to therein principally represents a dihydric alcohol and polyhydric alcohols having five or more hydroxyl groups are not disclosed. Moreover, a titanium-containing solution that contains a titanium compound, an aliphatic diol, and a polyhydric alcohol having three or more alcoholic hydroxyl groups is disclosed (patent document 6). The polyhydric alcohol having three or more alcoholic hydroxyl groups is used as a dissolution assistant for a purpose of increasing the solubility of a catalyst and thereby increasing the stability of a catalyst solution so as not to allow any precipitation to generate and a purpose of failing to have a bad influence on the quality of an aliphatic diol to be recovered and reused, and therefore it is basically different from embodiments of the present invention. Moreover, there is no disclosure about an effect of improving the color tone of a polymer to be obtained or the improvement in heat resistance and there is no concrete disclosure about mannitol. Aside from the above, a catalyst for polyester production that contains titanium oxide as a primary component and also contains a polyhydric alcohol is disclosed (patent document 7), but this is different from embodiments of the present invention because titanium oxide is generally low in reactivity, so that it is not able to react with a ligand, and it also is low in polymerization activity. Moreover, there is no concrete disclosure about mannitol.
While polyethylene terephthalate, one of the raw materials of which is terephthalic acid, is generally produced by performing an esterification reaction from terephthalic acid and ethylene glycol and a subsequent polycondensation reaction, it is presumed that it is possible to shorten the esterification reaction time and thereby inhibit coloring if a catalyst for polyester production is added before the esterification reaction. However, a titanium catalyst is generally unstable to water and therefore it reacts with water that is produced as a by-product of an esterification reaction. Thus, it forms a titanium oxide and loses its activity, resulting in the delay of an esterification reaction and a polycondensation reaction and eventually affording a deeply colored polymer.